(1) Field of the Invention
This invention relates to a digital signal transmission system. More particularly, it relates to a digital signal transmission system in which when the operating speed or bit rate of signals to be transmitted is unequal to the operating speed of a transmission line including repeaters etc., the signals are converted and synchronized with the operating speed of the transmission line and then the resulting signals are transmitted.
(2) Description of the Prior Art
In a prior-art digital signal transmission system, the bit rate of a transmission line is designed so as to be equal to the bit rate of digital signals to-be-transmitted. Accordingly, when the bit rate of digital signals is unequal to that of the transmission line, they cannot be directly transmitted.
Therefore, signal format converters for synchronizing the signals with the bit rate of the transmission line need to be disposed at sending and receiving terminals. In this case, buffer circuits are necessary. It is difficult, however, to perform the writing of inputs and the reading of outputs of the buffer circuit continuously, i.e., in real time. For this reason, signals which can be synchronized by disposing the buffer circuits have been limited to special digital signals, for example, signals of the start-stop method. Further, it has been considered impossible to multiplex a plurality of asynchronous signals. In general, accordingly, it is proposed to set the bit rate of the transmission line in conformity with the bit period of the signals to-be-transmitted or to set the bit period of the digital signals in conformity with the bit rate of the transmission line.
With the progress of the digital processing technology, however, sending terminals or channels which generate many kinds of digital signals of unequal bit rates have increased. Also in the transmission line, at the present time at which high-speed transmission has become possible with a transmission medium of very wide band, such as an optical fiber, if signals having arbitrary bit rates can be transmitted irrespective of the bit rate of the transmission line, the economical utilization of the transmission line can be achieved very advantageously.
It is also considered that if the signals can be transmitted in a multiplexed manner, the ease of use as the transmission line will be enhanced.